1. Field of the invention
This invention relates to a method of producing multifilament polyester yarns, by spinning in the molten state and winding at a high speed, and in particular, to a spinning method of a direct type in a single stage and filaments obtained thereby.
2. Prior art
Traditional continuous polyester filament production processes are usually of a two-stage type: a first so-called "spinning" stage wherein the polyester polymer is fed in the molten state into one or more dies through a suitable extruder, thereafter the filaments exiting the die orifices are cooled and solidified, and then are suitably oiled and towed by a spinning roller at a peripheral speed on the order of 1,000 meters/minute.
The yarn is then passed through a second stage, called the "stretching" stage, wherein using the speed differential between the first spinning roller and in the next, properly termed the stretching one, the yarn is subjected to elongation with respect to the starting length value which it had on exiting the first stage, on the order of 300% to 400%.
The two steps just described may take place on either of two separate apparata or a single apparatus; the former instance is that of separate spinning and stretching, the latter is that of continuous spinning and stretching. Modern production machinery usually operate in the continuous spinning and stretching mode and achieve picking speeds in the range of 3,000 to 3,500 meters/minute.
It has been already proposed in prior publications eliminate the stretching stage and picking directly at such speeds, and at even higher speeds, the polyester yarn.
Such prior publications, however, fail to fully solve the problems connected with the elimination of the stretching stage and consisting respectively of inadequate textile characteristics of the resulting yarns and difficulties which accompany operation of the picking machinery in the complete absence of spinning rollers. In some cases elimination of the stretching has been suggested but not of both the spinning rollers as usually employed, the presence of at least one of them being in all cases regarded as necessary, because by suitably controlling the peripheral speed of such a spinning roller relative to that at which picking is carried out, correct copping speeds can be achieved to form yarn cops which can be used in subsequent processing.
In other cases that spinning roller also serves, owing to its being heated, the function of a device adapted to fix the oncoming yarn from spinning to improve some ot its otherwise irregular characteristics, such as recovery.
Currently available are picking assemblies which operate at an operating speed in excess of 3,500 meters/minute as previously specified, and in particular which operate up to about 5,500 meters/minute without any particular trouble of a strictly mechanical nature.
With the aid of such picking machinery, therefore, it is highly desirable to have a processing method available which can avoid the need for using, to obtain suitable yarn packages for subsequent textile processing, at the same time deflection spinning rollers of any kind, and particularly of the heated type. This is because of the costs connected both with the purchase of such additional machinery and the burden connected with its operation.
Other prior publications indicate that in the speed range from 3,500 to 5,500 meters/ minute owing to phenomena connected with the molecular structure of yarns, in particular crystallinity and orientation (wherewith the yarn birefrigence value is associated), dye-taking ability is irregular. However, the suggestions proposed do not solve that problem.
In general, none of the available teachings in literature allow for operation of the spinning and picking lines in a single stage and without spinning rollers, it being satisfactory for industrial practice that the problems of yarn quality, breakages of the yarn during copping, and high rate of waste and rejected yarn connected with such productions.
Such problems are particularly enhanced in the instance of the production of yarns of so-called non-circular cross-section from a polymer lacking opacifier substance of the so-called lustre type.
The Applicant has unexpectedly found that the aforementioned problems can be solved and overcome, and that it is possible to obtain yarn with assigned properties and suitable for subsequent textile processing with a direct spinning process at picking speeds in the range from 3,500 to 5,500 meters/minute.